The invention concerns an apparatus for testing the air-permeability of lengths of textiles, in particular of substantially incompressible paper-machine felts or dryer fabrics, and includes a testing fork with rigid, mutually spaced and essentialy mutually parallel legs to the free ends of each of which mounted one pipe stub, at least one of said stubs being displaceable by means of a clamping device for clamping a length of fabric between the pipe stubs. The displaceable pipe stub is movable with respect to the associated leg in essentially coaxial manner and toward the other pipe stub. The other pipe stub is connected by an air pipe disposed largely in rectilinear manner, in the relevant leg to a motor-driven exhauster, both the motor and the exhauster being mounted in a frame joining the two legs. At least one control means for setting the flow rate, at least one vacuum gauge, preferably a first differential-pressure meter, and at least one air volumeter or airflow-rate meter being further provided in the air pipe for controlling and regulating the device.
Such a test apparatus is described in the German Gebrauchsmuster No. 82 16 624. It has proven itself by providing accurate and, most importantly, reproducible test values. However, spurious test results are frequently incurred when the air-permeability of a largely incompressible length of material, for instance a spiral fabric or a dryer fabric made of monofilamentary, synthetic or multilayer woven yarns, or dryer fabrics containing glass fibers, is measured. This is due to the edge seals being inadequate, and therefore spurious air can enter laterally into the pipe stubs which are connected to the air pipe.
It is therefore the object of the invention to so further develop the known testing apparatus that the entry of spurious air is prevented when measuring largely incompressible lengths of materials.
This problem is solved by the invention in that at least the pipe stub which is connected to the air pipe is designed as a double pipe stub having mutually coaxial inner and outer pipe parts. The air pipe is connected to the inner pipe part of the relevant double pipe stub and the outer pipe part is connected to a parallel pipe entering the air pipe and passing within the relevant leg between the air volumeter or airflow rate meter and the exhauster. A control valve to set the degree of vacuum is mounted in said parallel pipe, and the outer pipe part and the inner pipe part of the immobile double pipe stub are connected to a second differential-pressure meter.
This second differential-pressure meter regulates the control valve in the parallel pipe so that the pressures inside the inner pipe part and in the annular space between the inner and outer pipe parts are equal, that is, there is no pressure difference. Consequently no spurious air can enter the inner pipe part and thereby float to the volumeter or airflow rate meters, which would introduce a spurious component in the measurements. Thus, what is measured is the actual air moving through the cross-section of the inner pipe part and passing through the length of material.
Preferably, both pipe stubs are designed as double pipe stubs, the mutually opposite edges of the inner and outer pipe parts always being congruent. The clear spacing betwen the inner and outer pipe parts always being be at least 5 mm.
Appropriately, the inner and outer pipe parts are mutually joined in rigid manner by means of brackets and the like.
It is particularly advantageous to connect the control valve in the parallel pipe with a control motor which in turn is connected to a vacuum regulator connected to the second differential-pressure meter. Any pressure difference that might occur between the inner and outer pipe parts is thereby compensated far by a corresponding adjustment in the control valve. External intervention and monitoring are no longer required.
The invention furthermore provides aerodynamically rounded air intakes at the mobile pipe stub. This has not been the case to-date in the previously known testing apparatus and as a result, especially at the high air flows that occur when testing open fabrics, suctionjet contractions occur in the prior art devices, which reduce the effective testing cross-section and thereby introduce results which cannot be compared. This is prevented by the rounded air intake(s). Additionally, the mobile pipe stub has not flow-inhibiting or cross-section varying elements. Also, both pipe stubs and also the air pipe are provided with a polished inner surface. This step minimizes intake and transmission losses and provides an optimum load on the test surface.
The invention moreover provides that the mobile pipe stub is displaceable by means of a pantograph. More advantageously a linear kinematics is provided, which includes substantially linear guide elements consisting of a guide rod and a guide groove enclosing said rod.
In a further feature of the invention, provision is made for a motor generating a defined pressure to move one of the pipe stubs. A motor designed as a doubly acting hydraulic or pneumatic cylinder is especially well suited. The motor permits a continuous setting of the clamping force and furthermore assures that the set clamping force remains constant, whereby the samples always are subjected to the same clamping pressure. Appropriately, an analogue or digital display of the clamping pressure is provided on the mobile double pipe stub. The mobile double pipe stub can be connected by a bell crank to the hydraulic or pneumatic cylinder.
A further design of the invention provides that the pipe stubs can be exchanged as a set. This can be performed, for instance, by screwing them into fitted supports or by using a bayonet lock. In this manner, pipe stubs of different clear cross-sections can be used.
It is further advantageous to insert into all mutually facing end sides of the pipe stubs at least one, and preferably several concentric O-seals of circular or square cross-section. Thereby, a labyrinth effect is obtained, which contributes to preventing the laterial inflow of spurious air, especially at high air flow rates.
In order to uniformly load the test surface and to achieve the best aerodynamic effect, it is furthermore appropriate that the air pipe is coaxial with the inner pipe part of the double pipe stub connected to it and has a length at least twice said air pipe's diameter.
The invention further proposes mounting the volumeter or airflow rate meter in the straight end region of the suction pipe on the exhauster side. This arrangement provides an accurate measurement of the airflow or of the air volume because the long, straight pipe provides as a calming stretch.
Heat-based test instruments have been found to be especially suitable for measuring air flow rates. The hotwire anemometer and even more so a so-called hot-film sensor have been found especially suitable, the latter even more so because it is insensitive to entrained dust particles and impurities. These test instruments are characterized by a very high dynamic range of 1:100 and by very high accuracy. The entire measurement range of the test apparatus therefore can be covered by a single pick-up means. Alternatively, the air flow meter can be designed to be a third differential-pressure meter, in particular jointly with a nozzle or throttle inside the air channel. Obviously, the air flow meter should, in this case too, be connected to an analogue or digital test display.
In a further feature of the invention, the first differential-pressure meter is provided with a pressure tap in each of the two pipe stubs, i.e. at their inner pipe parts. Heretofore, only the vacuum in the exhaust line has been tapped. It is especially advantageous that the pipe stubs or their inner pipe parts comprise several--at least three--pressure taps distributed over the circumference and connected each by means of an annular channel which may be located in the inner pipe part(s), averaging being achieved thereby. For the same reason, the outer pipe part of the immobile double pipe stub should comprise several circumferentially distributed pressure taps.
The invention furthermore proposes hooking up at least one bypass line having a control valve mounted therein to the suction pipe between the air volumeter or air flow rate meter on one hand and the exhauster on the other. If a single bypass line is insufficient to cover the entire range of regulation, it may be advantageous to provide a larger bypass line with a coarse control valve and a smaller bypass line with a fine-control valve. This slitting-up makes it possible, on one hand, to almost completely erase the vacuum generated by the exhauster in order to assure that, even for the case of very dense samples, regulation shall still be operative. On the other hand, following approximate regulation by the coarse control valve in the larger bypass lie, very accurate control will be possible by means of the fine-control valve in the smaller bypass line, with the result being improved testing accuracy. Each of the coarse and fine control valves may be equipped with a setting motor, with the setting motor for the coarse-control valve being connected to a three-position control having an adjustable dead range. The vacuum will be kept constant within this dead range by the fine-control valve. To that end, the differential-pressure pickup and the control valve(s) are mounted in a control loop in order to set and keep constant the differential pressure. In this manner, any changes in pressure are henceforth prevented from affecting the test results as occurred in prior art devices.
The invention further provides that the testing fork, together with all of its parts, is mounted within one housing whereby it can be handled as one unit and acccordingly comprises a caster vehicle so that it can be moved into the sample. Also, a linear guide means can be provided which displaceably supports the test fork and which itself appropriately rests on a vehicle.
In order to measure the central area wide paper-machine felts and dryer fabrics also in the central area, the test fork includes an aperture at least 1.5 m long, with values of about 2 m long desirable.
Lastly, the invention proposes that the blower be in the form of an exhauster generating a vacuum of at least 2 mbars for an open test apparatus. This makes possible a test procedure wherein the test-apparatus loss is eliminated. To that end, the airflow of the open test apparatus at a defined differential pressure is first measured. Then the sample is clamped in place and the air flow is measured at the same differential pressure. The airflow to be assigned to the sample is computed from the difference between the two test values. As a result, values of air permeability can be computed.